The invention relates to a process for assembling a cathode for electron gun, which process is more particularly suitable for a so-called impregnated cathode.
An impregnated cathode comprises an emissive part in the form of a porous substance made of a refractory material (tungsten, molybdenum, rhenium) impregnated with an electron emitting material (barium, strontium, calcium, aluminium, scandium, osmium, etc.), of a metal cup into which the porous emissive substance is inserted and of a metal sleeve manufactured from a refractory material such as molybdenum, tantalum or tungsten, which sleeve is also referred to as the cathode skirt. The cup containing the emissive substance is disposed at one of the ends of the sleeve. Inserted at the other end of the sleeve is the filament for heating the cathode, which filament raises the said emissive part of the cathode to a temperature of around 1000xc2x0 C. during its operation.
The cup is generally made from a refractory material such as tantalum or molybdenum; it makes it possible to insulate the emissive part of the heating filament so that the materials emitted during the operation of the cathode by the emissive part cannot pollute the filament and destroy the insulating coating of the latter, causing the destruction of the said filament, and consequently that of the cathode itself.
The impregnated body constituting the emissive substance of the cathode must remain in contact with the metal surface of the cup so as to optimize the thermal efficiency of the cathode. Poor contact, leaving gaps between the opposite surfaces of the impregnated body and of the cup, reduces this thermal efficiency, this having the consequence of reducing the current density which the said cathode can emit. Moreover, poor contact between the body and the cup will cause dispersions of unacceptable emission characteristic when these cathodes are for example inserted in threes into electron guns for colour cathode-ray tube. Now, owing in fact to the materials used, it is difficult to immobilize the impregnated porous body in its cup either by soldering or by welding, in a reliable manner with sufficient repeatability.
Several solutions are described in the state of the art for solving this difficulty. Patent EP 272881 describes the use of Rhenium wires disposed between the body and the cup so as to improve the welding of these two elements. U.S. Pat. No. 5,128,584 describes a process in which the impregnation of the porous metal body is carried out after the operation of laser welding of the body and of the cup. Finally, the use of an intermediate piece intended to play the role of an interface improving the welding between the body and the cup is described in the publication entitled xe2x80x9cTemperature and cutoff stabilization of impregnated cathodes, published in the proceedings of the xe2x80x9cSID 96 digestxe2x80x9d for the conferences of the xe2x80x9cSociety for Information Displayxe2x80x9d which were held in 1996, as well as in patent application EP 798 758.
All these solutions are expensive and complex to implement especially when the cathodes have very small dimensions, for example of the order of a millimetre for the diameter of the emissive body.
The invention proposes a simple and economic solution for ensuring perfect retention of the emissive body in its cup devoid of the drawbacks resulting from the use of the techniques described in the prior art.
To achieve this object, the invention relates to a process for assembling a cathode for electron gun, which cathode comprising a body of emissive material, a cup, comprising a bottom and a lateral wall, into which cup is inserted the body of emissive material, and a substantially cylindrical metal skirt, the said process comprising the following successive steps:
insertion of the cup into one of the open ends of the metal skirt
welding of the cup to the skirt
crimping of the body/cup/skirt assembly by lateral squeezing at the level of the weld zone in such a way as to cause an indent-like deformation of the lateral face of the body opposite the lateral wall of the cup.